Protein transport and secretion are essential for cellular function. Protein transport is mediated by a signal peptide located at the amino terminus of the protein to be transported or secreted. The signal peptide is comprised of about ten to twenty hydrophobic amino acids which target the nascent protein from the ribosome to a particular membrane bound compartment such as the endoplasmic reticulum (ER). Proteins targeted to the ER may either proceed through the secretory pathway or remain in any of the secretory organelles such as the ER, Golgi apparatus, or lysosomes. Proteins that transit through the secretory pathway are either secreted into the extracellular space or retained in the plasma membrane. Proteins that are retained in the plasma membrane contain one or more transmembrane domains, each comprised of about 20 hydrophobic amino acid residues. Secreted proteins are generally synthesized as inactive precursors that are activated by post-translational processing events during transit through the secretory pathway. Such events include glycosylation, proteolysis, and removal of the signal peptide by a signal peptidase. Other events that may occur during protein transport include chaperone-dependent unfolding and folding of the nascent protein and interaction of the protein with a receptor or pore complex. Examples of secreted proteins with amino terminal signal peptides are discussed below and include proteins with important roles in cell-to-cell signaling. Such proteins include transmembrane receptors and cell surface markers, extracellular matrix molecules, cytokines, hormones, growth and differentiation factors, enzymes, neuropeptides, vasomediators, cell surface markers, and antigen recognition molecules. (Reviewed in Alberts, B. et al. (1994) Molecular Biology of The Cell, Garland Publishing, New York, N.Y., pp. 557-560, 582-592.)
Cell surface markers include cell surface antigens identified on leukocytic cells of the immune system. These antigens have been identified using systematic, monoclonal antibody (mAb)-based “shot gun” techniques. These techniques have resulted in the production of hundreds of mAbs directed against unknown cell surface leukocytic antigens. These antigens have been grouped into “clusters of differentiation” based on common immunocytochemical localization patterns in various differentiated and undifferentiated leukocytic cell types. Antigens in a given cluster are presumed to identify a single cell surface protein and are assigned a “cluster of differentiation” or “CD” designation. Some of the genes encoding proteins identified by CD antigens have been cloned and verified by standard molecular biology techniques. CD antigens have been characterized as both transmembrane proteins and cell surface proteins anchored to the plasma membrane via covalent attachment to fatty acid-containing glycolipids such as glycosylphosphatidylinositol (GPI). (Reviewed in Barclay, A. N. et al. (1995) The Leucocyte Antigen Facts Book, Academic Press, San Diego, Calif., pp. 17-20.)
Matrix proteins (MPs) are transmembrane and extracellular proteins which function in formation, growth, remodeling, and maintenance of tissues and as important mediators and regulators of the inflammatory response. The expression and balance of MPs may be perturbed by biochemical changes that result from congenital, epigenetic, or infectious diseases. In addition, MPs affect leukocyte migration, proliferation, differentiation, and activation in the immune response. MPs are frequently characterized by the presence of one or more domains which may include collagen-like domains, EGF-like domains, immunoglobulin-like domains, and fibronectin-like domains. In addition, MPs may be heavily glycosylated and may contain an Arginine-Glycine-Aspartate (RGD) tripeptide motif which may play a role in adhesive interactions. MPs include extracellular proteins such as fibronectin, collagen, galectin, vitronectin and its proteolytic derivative somatomedin B; and cell adhesion receptors such as cell adhesion molecules (CAMs), cadherins, and integrins. (Reviewed in Ayad, S. et al. (1994) The Extracellular Matrix Facts Book, Academic Press, San Diego, Calif., pp. 2-16; Ruoslahti, E. (1997) Kidney Int. 51:1413-1417; Sjaastad, M. D. and Nelson, W. J. (1997) BioEssays 19:47-55.)
Peroxidasin is a Drosophila protein that contains both peroxidase and extracellular matrix motifs. The 1512 amino acid peroxidasin protein contains a peroxidase domain homologous to human myeloperoxidase and eosiniphil peroxidase, as well as six leucine-rich repeats, four immunoglobulin domains, and a region of thrombospondin/procollagen homology. Peroxidasin is secreted by hemocytes as they spread throughout the developing Drosophila embryo. The protein is thought to function in extracellular matrix consolidation, phagocytosis, and defense (Nelson, R. E. (1994) EMBO J. 13:3438-3447). A human homolog of the Drosophila peroxidasin gene was recently found to be upregulated in a colon cancer cell line undergoing p53 tumor suppressor-dependent apoptosis, and thus may play a role in the mechanisms of p53-dependent apoptosis (Horikoshi, N. et al. (1999) Biochem. Biophy. Res. Commun. 261:864-869).
Mucins are highly glycosylated glycoproteins that are the major structural component of the mucus gel. The physiological functions of mucins are cytoprotection, mechanical protection, maintenance of viscosity in secretions, and cellular recognition. MUC6 is a human gastric mucin that is also found in gall bladder, pancreas, seminal vesicles, and female reproductive tract (Toribara, N. W. et al. (1997) J. Biol. Chem. 272:16398-16403). The MUC6 gene has been mapped to human chromosome 11 (Toribara, N. W. et al. (1993) J. Biol. Chem. 268:5879-5885). Hemomucin is a novel Drosophila surface mucin that may be involved in the induction of antibacterial effector molecules (Theopold, U. et al. (1996) J. Biol. Chem. 217:12708-12715).
Tuftelins are one of four different enamel matrix proteins that have been identified so far. The other three known enamel matrix proteins are the amelogenins, enamelin and ameloblastin. Assembly of the enamel extracellular matrix from these component proteins is believed to be critical in producing a matrix competent to undergo mineral replacement. (Paine, C. T. et al. (1998) Connect Tissue Res. 38:257-267). Tuftelin mRNA has been found to be expressed in human ameloblastoma tumor, a non-mineralized odontogenic tumor (Deutsch, D. et al. (1998) Connect. Tissue Res. 39:177-184).
Olfactomedin-related proteins are extracellular matrix, secreted glycoproteins with conserved C-terminal motifs. They are expressed in a wide variety of tissues and in broad range of species, from Caenorhabditis elegans to Homo sapiens. Olfactomedin-related proteins comprise a gene family with at least 5 family members in humans. One of the five, TIGR/myocilin protein, is expressed in the eye and is associated with the pathogenesis of glaucoma (Kulkarni, N. H. et al. (2000) Genet. Res. 76:41-50). Research by Yokoyama et al. (1996) found a 135-amino acid protein, termed AMY, having 96% sequence identity with rat neuronal olfactomedin-releated ER localized protein in a neuroblastoma cell line cDNA library, suggesting an essential role for AMY in nerve tissue (Yokoyama, M. et al. (1996) DNA Res. 3:311-320). Neuron-specific olfactomedin-related glycoproteins isolated from rat brain cDNA libraries show strong sequence similarity with olfactomedin. This similarity is suggestive of a matrix-related function of these glycoproteins in neurons and neurosecretory cells (Danielson, P. E. et al. (1994) J. Neurosci. Res. 38:468-478).
Mac-2 binding protein is a 90-kD serum protein (90K), a secreted glycoprotein isolated from both the human breast carcinoma cell line SK-BR-3, and human breast milk. It specifically binds to a human macrophage-associated lectin, Mac-2. Structurally, the mature protein is 567 amino acids in length and is proceeded by an 18-amino acid leader. There are 16 cysteines and seven potential N-linked glycosylation sites. The first 106 amino acids represent a domain very similar to an ancient protein superfamily defined by a macrophage scavenger receptor cysteine-rich domain (Koths, K. et al. (1993) J. Biol. Chem. 268:14245-14249). 90K is elevated in the serum of subpopulations of AIDS patients and is expressed at varying levels in primary tumor samples and tumor cell lines. Ullrich et al. (1994) have demonstrated that 90K stimulates host defense systems and can induce interleukin-2 secretion. This immune stimulation is proposed to be a result of oncogenic transformation, viral infection or pathogenic invasion (Ullrich, A., et al. (1994) J. Biol. Chem. 269:18401-18407).
Semaphorins are a large group of axonal guidance molecules consisting of at least 30 different members and are found in vertebrates, invertebrates, and even certain viruses. All semaphorins contain the sema domain which is approximately 500 amino acids in length. Neuropilin, a semaphorin receptor, has been shown to promote neurite outgrowth in vitro. The extracellular region of neuropilins consists of three different domains: CUB, discoidin, and MAM domains. The CUB and the MAM motifs of neuropilin have been suggested to have roles in protein-protein interactions and are thought to be involved in the binding of semaphorins through the sema and the C-terminal domains (reviewed in Raper, J. A. (2000) Curr. Opin. Neurobiol. 10:88-94). Plexins are neuronal cell surface molecules that mediate cell adhesion via a homophilic binding mechanism in the presence of calcium ions. Plexins have been shown to be expressed in the receptors and neurons of particular sensory systems (Ohta, K. et al. (1995) Cell 14:1189-1199). There is evidence that suggests that some plexins function to control motor and CNS axon guidance in the developing nervous system. Plexins, which themselves contain complete semaphorin domains, may be both the ancestors of classical semaphorins and binding partners for semaphorins (Winberg, M. L. et al (1998) Cell 95:903-916).
Human pregnancy-specific beta 1-glycoprotein (PSG) is a family of closely related glycoproteins of molecular weights of 72 KDa, 64 KDa, 62 KDa, and 54 KDa. Together with the carcinoembryonic antigen, they comprise a subfamily within the immunoglobulin superfamily (Plouzek, C. A. and Chou, J. Y. (1991) Endocrinology 129:950-958) Different subpopulations of PSG have been found to be produced by the trophoblasts of the human placenta, and the amnionic and chorionic membranes (Plouzek, C. A. et al. (1993) Placenta 14:277-285).
Autocrine motility factor (AMF) is one of the motility cytokines regulating tumor cell migration; therefore identification of the signaling pathway coupled with it has critical importance. Autocrine motility factor receptor (AMFR) expression has been found to be associated with tumor progression in thymoma (Ohta Y. et al. (2000) Int. J. Oncol. 17:259-264). AMFR is a cell surface glycoprotein of molecular weight 78 KDa.
Hormones are secreted molecules that travel through the circulation and bind to specific receptors on the surface of, or within, target cells. Although they have diverse biochemical compositions and mechanisms of action, hormones can be grouped into two categories. One category includes small lipophilic hormones that diffuse through the plasma membrane of target cells, bind to cytosolic or nuclear receptors, and form a complex that alters gene expression. Examples of these molecules include retinoic acid, thyroxine, and the cholesterol-derived steroid hormones such as progesterone, estrogen, testosterone, cortisol, and aldosterone. The second category includes hydrophilic hormones that function by binding to cell surface receptors that transduce signals across the plasma membrane. Examples of such hormones include amino acid derivatives such as catecholamines (epinephrine, norepinephrine) and histamine, and peptide hormones such as glucagon, insulin, gastrin, secretin, cholecystokinin, adrenocorticotropic hormone, follicle stimulating hormone, luteinizing hormone, thyroid stimulating hormone, and vasopressin. (See, for example, Lodish et al. (1995) Molecular Cell Biology, Scientific American Books Inc., New York, N.Y., pp. 856-864.)
Pro-opiomelanocortin (POMC) is the precursor polypeptide of corticotropin (ACTH), a hormone synthesized by the anterior pituitary gland, which functions in the stimulation of the adrenal cortex. POMC is also the precursor polypeptide of the hormone beta-lipotropin (beta-LPH). Each hormone includes smaller peptides with distinct biological activities: alpha-melanotropin (alpha-MSH) and corticotropin-like intermediate lobe peptide (CLIP) are formed from ACTH; gamma-lipotropin (gamma-LPH) and beta-endorphin are peptide components of beta-LPH; while beta-MSH is contained within gamma-LPH. Adrenal insufficiency due to ACTH deficiency, resulting from a genetic mutation in exons 2 and 3 of POMC results in an endocrine disorder characterized by early-onset obesity, adrenal insufficiency, and red hair pigmentation (Chretien, M. et al. (1979) Canad. J. Biochem. 57:1111-1121; Krude, H. et al. (1998) Nature Genet. 19:155-157; Online Mendelian Inheritance in Man (OMIM) 176830).
Growth and differentiation factors are secreted proteins which function in intercellular communication. Some factors require oligomerization or association with membrane proteins for activity. Complex interactions among these factors and their receptors trigger intracellular signal transduction pathways that stimulate or inhibit cell division, cell differentiation, cell signaling, and cell motility. Most growth and differentiation factors act on cells in their local environment (paracrine signaling). There are three broad classes of growth and differentiation factors. The first class includes the large polypeptide growth factors such as epidermal growth factor, fibroblast growth factor, transforming growth factor, insulin-like growth factor, and platelet-derived growth factor. The second class includes the hematopoietic growth factors such as the colony stimulating factors (CSFs). Hematopoietic growth factors stimulate the proliferation and differentiation of blood cells such as B-lymphocytes, T-lymphocytes, erythrocytes, platelets, eosinophils, basophils, neutrophils, macrophages, and their stem cell precursors. The third class includes small peptide factors such as bombesin, vasopressin, oxytocin, endothel in, transferrin, angiotensin II, vasoactive intestinal peptide, and bradykinin, which function as hormones to regulate cellular functions other than proliferation.
Growth and differentiation factors play critical roles in neoplastic transformation of cells in vitro and in tumor progression in vivo. Inappropriate expression of growth factors by tumor cells may contribute to vascularization and metastasis of tumors. During hematopoiesis, growth factor misregulation can result in anemias, leukemias, and lymphomas. Certain growth factors such as interferon are cytotoxic to tumor cells both in vivo and in vitro. Moreover, some growth factors and growth factor receptors are related both structurally and functionally to oncoproteins. In addition, growth factors affect transcriptional regulation of both proto-oncogenes and oncosuppressor genes. (Reviewed in Pimentel, E. (1994) Handbook of Growth Factors, CRC Press, Ann Arbor, Mich., pp. 1-9.)
The Slit protein, first identified in Drosophila, is critical in central nervous system midline formation and potentially in nervous tissue histogenesis and axonal pathfinding. Itoh et al. ((1998) Brain Res. Mol. Brain. Res. 62:175-186) have identified mammalian homologues of the slit gene (hurnan Slit-1, Slit-2, Slit-3 and rat Slit-1). The encoded proteins are putative secreted proteins containing EGF-like motifs and leucine-rich repeats, both of which are conserved protein-protein interaction domains. Slit-1, -2, and -3 mRNAs are expressed in the brain, spinal cord, and thyroid, respectively (Itoh, A. et al., supra). The Slit family of proteins are indicated to be functional ligands of glypican-1 in nervous tissue and it is suggested that their interactions may be critical in certain stages during central nervous system histogenesis (Liang, Y. et al., (1999) J. Biol. Chem. 274:17885-17892).
Neuropeptides and vasomediators (NP/VM) comprise a large family of endogenous signaling molecules. Included in this family are neuropeptides and neuropeptide hormones such as bombesin, neuropeptide Y, neurotensin, neuromedin N, melanocortins, opioids, galanin, somatostatin, tachykinins, urotensin II and related peptides involved in smooth muscle stimulation, vasopressin, vasoactive intestinal peptide, and circulatory system-borne signaling molecules such as angiotensin, complement, calcitonin, endothelins, formyl-methionyl peptides, glucagon, cholecystokinin and gastrin. NP/VMs can transduce signals directly, modulate the activity or release of other neurotransmitters and hormones, and act as catalytic enzymes in cascades. The effects of NP/VMs range from extremely brief to long-lasting. (Reviewed in Martin, C. R. et al. (1985) Endocrine Physiology, Oxford University Press, New York, N.Y., pp. 57-62.)
NP/VMs are involved in numerous neurological and cardiovascular disorders. For example, neuropeptide Y is involved in hypertension, congestive heart failure, affective disorders, and appetite regulation. Somatostatin inhibits secretion of growth hormone and prolactin in the anterior pituitary, as well as inhibiting secretion in intestine, pancreatic acinar cells, and pancreatic beta-cells. A reduction in somatostatin levels has been reported in Alzheimer's disease and Parkinson's disease. Vasopressin acts in the kidney to increase water and sodium absorption, and in higher concentrations stimulates contraction of vascular smooth muscle, platelet activation, and glycogen breakdown in the liver. Vasopressin and its analogues are used clinically to treat diabetes insipidus. Endothelin and angiotensin are involved in hypertension, and drugs, such as captopril, which reduce plasma levels of angiotensin, are used to reduce blood pressure (Watson, S. and S. Arkinstall (1994) The G-protein Linked Receptor Facts Book, Academic Press, San Diego Calif., pp. 194; 252; 284; 55; 111).
Neuropeptides have also been shown to have roles in nociception (pain). Vasoactive intestinal peptide appears to play an important role in chronic neuropathic pain. Nociceptin, an endogenous ligand for the opioid receptor-like 1 receptor, is thought to have a predominantly anti-nociceptive effect, and has been shown to have analgesic properties in different animal models of tonic oF chronic pain (Dickinson, T. and Fleetwood-Walker, S. M. (1998) Trends Pharmacol. Sci. 19:346-348).
Other proteins that contain signal peptides include secreted proteins with enzymatic activity. Such activity includes, for example, oxidoreductase/dehydrogenase activity, transferase activity, hydrolase activity, lyase activity, isomerase activity, or ligase activity. For example, matrix metalloproteinases are secreted hydrolytic enzymes that degrade the extracellular matrix and thus play an important role in tumor metastasis, tissue morphogenesis, and arthritis (Reponen, P. et al. (1995) Dev. Dyn. 202:388-396; Firestein, G. S. (1992) Curr. Opin. Rheumatol. 4:348-354; Ray, J. M. and Stetler-Stevenson, W. G. (1994) Eur. Respir. J. 7:2062-2072; and Mignatti, P. and Rifkin, D. B. (1993) Physiol. Rev. 73:161-195). Additional examples are the acetyl-CoA synthetases which activate acetate for use in lipid synthesis or energy generation (Luong, A. et al. (2000) J. Biol. Chem. 275:26458-26466). The result of acetyl-CoA synthetase activity is the formation of acetyl-CoA from acetate and CoA. Acetyl-CoA sythetases share a region of sequence similarity identified as the AMP-binding domain signature. Acetyl-CoA synthetase has been shown to be associated with hypertension (H. Toh (1991) Protein Seq. Data Anal. 4:111-117; and Iwai, N. et al., (1994) Hypertension 23:375-380).
A number of isomerases catalyze steps in protein folding, phototransduction, and various anabolic and catabolic pathways. One class of isomerases is known as peptidyl-prolyl cis-trans isomerases (PPIases). PPIases catalyze the cis to trans isomerization of certain proline imidic bonds in proteins. Two families of PPIases are the FK506 binding proteins (FKBPs), and cyclophilins (CyPs). FKBPs bind the potent immunosuppressants FK506 and rapamycin, thereby inhibiting signaling pathways in T-cells. Specifically, the PPIase activity of FKBPs is inhibited by binding of FK506 or rapamycin. There are five members of the FKBP family which are named according to their calculated molecular masses (FKBP12, FKBP13, FKBP25, FKBP52, and FKBP65), and localized to different regions of the cell where they associate with different protein complexes (Coss, M. et al. (1995) J. Biol. Chem. 270:29336-29341; Schreiber, S. L. (1991) Science 251:283-287).
The peptidyl-prolyl isomerase activity of CyP may be part of the signaling pathway that leads to T-cell activation. CyP isomerase activity is associated with protein folding and protein trafficking, and may also be involved in assembly/disassembly of protein complexes and regulation of protein activity. For example, in Drosophila, the CyP NinaA is required for correct localization of rhodopsins, while a mammalian CyP (Cyp40) is part of the Hsp90/Hsc70 complex that binds steroid receptors. The mammalian CypA has been shown to bind the gag protein from human immunodeficiency virus 1 (HIV-1), an interaction that can be inhibited by cyclosporin. Since cyclosporin has potent anti-HIV-1 activity, CypA may play an essential function in HIV-1 replication. Finally, Cyp40 has been shown to bind and inactivate the transcription factor c-Myb, an effect that is reversed by cyclosporin. This effect implicates CyPs in the regulation of transcription, transformation, and differentiation (Bergsma, D. J. et al (1991) J. Biol. Chem. 266:23204-23214; Hunter, T. (1998) Cell 92: 141-143; and Leverson, J. D. and Ness, S. A. (1998) Mol. Cell. 1:203-211).
Gamma-carboxyglutamic acid (Gla) proteins rich in proline (PRGPs) are members of a family of vitamin K-dependent single-pass integral membrane proteins. These proteins are characterized by an extracellular amino terminal domain of approximately 45 amino acids rich in Gla. The intracellular carboxyl terminal region contains one or two copies of the sequence PPXY, a motif present in a variety of proteins involved in such diverse cellular functions as signal transduction, cell cycle progression, and protein turnover (Kulman, J. D. et al., (2001) Proc. Natl. Acad. Sci. U.S.A. 98:1370-1375). The process of post-translational modification of glutamic residues to form Gla is Vitamin K-dependent carboxylation. Proteins which contain Gla include plasma proteins involved in blood coagulation. These proteins are prothrombin, proteins C, S, and Z, and coagulation factors VII, IX, and X. Osteocalcin (bone-Gla protein, BGP) and matrix Gla-protein (MGP) also contain Gla (Friedman, P. A., and C. T. Przysiecki (1987) Int. J. Biochem. 19:1-7; C. Vermeer (1990) Biochem. J. 266:625-636).
The Drosophila sp. gene crossveinless 2 is characterized as having a putative signal or transmembrane sequence, and a partial Von Willebrand Factor D domain similar to those domains known to regulate the formation of intramolecular and intermolecular bonds and five cysteine-rich domains, known to bind BMP-like (bone morphogenetic proteins) ligands. These features suggest that crossveinless 2 may act extracelluarly or in the secretory pathway to directly potentiate ligand signaling and hence, involvement in the BMP-like signaling pathway known to play a role in vein specification (Conley, C. A. et al., (2000) Development 127:3947-3959). The dorsal-ventral patterning in both vertebrate and Drosophila embryos requires a conserved system of extracellular proteins to generate a positional informational gradient.
The discovery of new secreted proteins, and the polynucleotides encoding them, satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention, and treatment of cell proliferative, autoimmune/inflammatory, cardiovascular, neurological, and developmental disorders, and in the assessment of the effects of exogenous compounds on the expression of nucleic acid and amino acid sequences of secreted proteins.